Rollup merge of rust-lang#98028 - aticu:master, r=estebank

Add E0789 as more specific variant of E0283

Fixes rust-lang#81701

I think this should be good to go, there are only two things where I am somewhat unsure:
- Is there a better way to get the fully-qualified path for the suggestion? I tried `self.tcx.def_path_str`, but that didn't seem to always give a correct path for the context.
- Should all this be extracted into it's own method or is it fine where it is?

r? `@estebank`

compiler/rustc_error_codes/src/error_codes.rs

@@ -492,6 +492,7 @@ E0785: include_str!("./error_codes/E0785.md"),
 E0786: include_str!("./error_codes/E0786.md"),
 E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
+E0789: include_str!("./error_codes/E0789.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0283.md

@@ -3,48 +3,27 @@ An implementation cannot be chosen unambiguously because of lack of information.
 Erroneous code example:
 
 ```compile_fail,E0283
-trait Generator {
-    fn create() -> u32;
-}
-
-struct Impl;
-
-impl Generator for Impl {
-    fn create() -> u32 { 1 }
-}
-
-struct AnotherImpl;
+struct Foo;
 
-impl Generator for AnotherImpl {
-    fn create() -> u32 { 2 }
+impl Into<u32> for Foo {
+    fn into(self) -> u32 { 1 }
 }
 
-fn main() {
-    let cont: u32 = Generator::create();
-    // error, impossible to choose one of Generator trait implementation
-    // Should it be Impl or AnotherImpl, maybe something else?
-}
+let foo = Foo;
+let bar: u32 = foo.into() * 1u32;
 ```
 
 This error can be solved by adding type annotations that provide the missing
-information to the compiler. In this case, the solution is to use a concrete
-type:
+information to the compiler. In this case, the solution is to specify the
+trait's type parameter:
 
 ```
-trait Generator {
-    fn create() -> u32;
-}
-
-struct AnotherImpl;
+struct Foo;
 
-impl Generator for AnotherImpl {
-    fn create() -> u32 { 2 }
+impl Into<u32> for Foo {
+    fn into(self) -> u32 { 1 }
 }
 
-fn main() {
-    let gen1 = AnotherImpl::create();
-
-    // if there are multiple methods with same name (different traits)
-    let gen2 = <AnotherImpl as Generator>::create();
-}
+let foo = Foo;
+let bar: u32 = Into::<u32>::into(foo) * 1u32;
 ```

compiler/rustc_error_codes/src/error_codes/E0789.md

@@ -0,0 +1,47 @@
+You need to specify a specific implementation of the trait in order to call the
+method.
+
+Erroneous code example:
+
+```compile_fail,E0789
+trait Generator {
+    fn create() -> u32;
+}
+
+struct Impl;
+
+impl Generator for Impl {
+    fn create() -> u32 { 1 }
+}
+
+struct AnotherImpl;
+
+impl Generator for AnotherImpl {
+    fn create() -> u32 { 2 }
+}
+
+let cont: u32 = Generator::create();
+// error, impossible to choose one of Generator trait implementation
+// Should it be Impl or AnotherImpl, maybe something else?
+```
+
+This error can be solved by adding type annotations that provide the missing
+information to the compiler. In this case, the solution is to use a concrete
+type:
+
+```
+trait Generator {
+    fn create() -> u32;
+}
+
+struct AnotherImpl;
+
+impl Generator for AnotherImpl {
+    fn create() -> u32 { 2 }
+}
+
+let gen1 = AnotherImpl::create();
+
+// if there are multiple methods with same name (different traits)
+let gen2 = <AnotherImpl as Generator>::create();
+```

compiler/rustc_middle/src/ty/trait_def.rs

@@ -74,6 +74,10 @@ impl TraitImpls {
     pub fn blanket_impls(&self) -> &[DefId] {
         self.blanket_impls.as_slice()
     }
+
+    pub fn non_blanket_impls(&self) -> &FxIndexMap<SimplifiedType, Vec<DefId>> {
+        &self.non_blanket_impls
+    }
 }
 
 impl<'tcx> TraitDef {

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2104,6 +2104,98 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                         );
                     }
                 }
+
+                if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
+                    (body_id, subst.map(|subst| subst.unpack()))
+                {
+                    struct FindExprBySpan<'hir> {
+                        span: Span,
+                        result: Option<&'hir hir::Expr<'hir>>,
+                    }
+
+                    impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
+                        fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+                            if self.span == ex.span {
+                                self.result = Some(ex);
+                            } else {
+                                hir::intravisit::walk_expr(self, ex);
+                            }
+                        }
+                    }
+
+                    let mut expr_finder = FindExprBySpan { span, result: None };
+
+                    expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
+
+                    if let Some(hir::Expr {
+                        kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
+                    ) = expr_finder.result
+                        && let [
+                            ..,
+                            trait_path_segment @ hir::PathSegment {
+                                res: Some(rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id)),
+                                ..
+                            },
+                            hir::PathSegment {
+                                ident: assoc_item_name,
+                                res: Some(rustc_hir::def::Res::Def(_, item_id)),
+                                ..
+                            }
+                        ] = path.segments
+                        && data.trait_ref.def_id == *trait_id
+                        && self.tcx.trait_of_item(item_id) == Some(*trait_id)
+                        && !self.is_tainted_by_errors()
+                    {
+                        let (verb, noun) = match self.tcx.associated_item(item_id).kind {
+                            ty::AssocKind::Const => ("refer to the", "constant"),
+                            ty::AssocKind::Fn => ("call", "function"),
+                            ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
+                        };
+
+                        // Replace the more general E0283 with a more specific error
+                        err.cancel();
+                        err = self.tcx.sess.struct_span_err_with_code(
+                            span,
+                            &format!(
+                                "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
+                             ),
+                            rustc_errors::error_code!(E0789),
+                        );
+
+                        if let Some(local_def_id) = data.trait_ref.def_id.as_local()
+                            && let Some(hir::Node::Item(hir::Item { ident: trait_name, kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs), .. })) = self.tcx.hir().find_by_def_id(local_def_id)
+                            && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
+                            err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
+                        }
+
+                        err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
+
+                        let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
+
+                        if trait_impls.blanket_impls().is_empty()
+                            && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
+                            && let Some(impl_def_id) = impl_ty.def() {
+                            let message = if trait_impls.non_blanket_impls().len() == 1 {
+                                "use the fully-qualified path to the only available implementation".to_string()
+                            } else {
+                                format!(
+                                    "use a fully-qualified path to a specific available implementation ({} found)",
+                                    trait_impls.non_blanket_impls().len()
+                                )
+                            };
+
+                            err.multipart_suggestion(
+                                message,
+                                vec![
+                                    (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
+                                    (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
+                                ],
+                                Applicability::MaybeIncorrect
+                            );
+                        }
+                    }
+                };
+
                 err
             }
 

src/test/ui/associated-consts/issue-63496.rs

@@ -2,8 +2,8 @@ trait A {
     const C: usize;
 
     fn f() -> ([u8; A::C], [u8; A::C]);
-    //~^ ERROR: type annotations needed
-    //~| ERROR: type annotations needed
+    //~^ ERROR: E0789
+    //~| ERROR: E0789
 }
 
 fn main() {}

src/test/ui/associated-consts/issue-63496.stderr

@@ -1,27 +1,21 @@
-error[E0283]: type annotations needed
+error[E0789]: cannot refer to the associated constant on trait without specifying the corresponding `impl` type
   --> $DIR/issue-63496.rs:4:21
    |
+LL |     const C: usize;
+   |     --------------- `A::C` defined here
+LL |
 LL |     fn f() -> ([u8; A::C], [u8; A::C]);
-   |                     ^^^^
-   |                     |
-   |                     cannot infer type
-   |                     help: use the fully qualified path to an implementation: `<Type as A>::C`
-   |
-   = note: cannot satisfy `_: A`
-   = note: associated constants cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
+   |                     ^^^^ cannot refer to the associated constant of trait
 
-error[E0283]: type annotations needed
+error[E0789]: cannot refer to the associated constant on trait without specifying the corresponding `impl` type
   --> $DIR/issue-63496.rs:4:33
    |
+LL |     const C: usize;
+   |     --------------- `A::C` defined here
+LL |
 LL |     fn f() -> ([u8; A::C], [u8; A::C]);
-   |                                 ^^^^
-   |                                 |
-   |                                 cannot infer type
-   |                                 help: use the fully qualified path to an implementation: `<Type as A>::C`
-   |
-   = note: cannot satisfy `_: A`
-   = note: associated constants cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
+   |                                 ^^^^ cannot refer to the associated constant of trait
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0283`.
+For more information about this error, try `rustc --explain E0789`.

src/test/ui/associated-item/issue-48027.rs

@@ -1,6 +1,6 @@
 trait Bar {
     const X: usize;
-    fn return_n(&self) -> [u8; Bar::X]; //~ ERROR: type annotations needed
+    fn return_n(&self) -> [u8; Bar::X]; //~ ERROR: E0789
 }
 
 impl dyn Bar {} //~ ERROR: the trait `Bar` cannot be made into an object

src/test/ui/associated-item/issue-48027.stderr

@@ -13,19 +13,15 @@ LL |     const X: usize;
    |           ^ ...because it contains this associated `const`
    = help: consider moving `X` to another trait
 
-error[E0283]: type annotations needed
+error[E0789]: cannot refer to the associated constant on trait without specifying the corresponding `impl` type
   --> $DIR/issue-48027.rs:3:32
    |
+LL |     const X: usize;
+   |     --------------- `Bar::X` defined here
 LL |     fn return_n(&self) -> [u8; Bar::X];
-   |                                ^^^^^^
-   |                                |
-   |                                cannot infer type
-   |                                help: use the fully qualified path to an implementation: `<Type as Bar>::X`
-   |
-   = note: cannot satisfy `_: Bar`
-   = note: associated constants cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`
+   |                                ^^^^^^ cannot refer to the associated constant of trait
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0038, E0283.
+Some errors have detailed explanations: E0038, E0789.
 For more information about an error, try `rustc --explain E0038`.

src/test/ui/associated-types/associated-types-unconstrained.rs

@@ -12,5 +12,5 @@ impl Foo for isize {
 
 pub fn main() {
     let x: isize = Foo::bar();
-    //~^ ERROR type annotations needed
+    //~^ ERROR E0789
 }

src/test/ui/associated-types/associated-types-unconstrained.stderr

@@ -1,11 +1,12 @@
-error[E0283]: type annotations needed
+error[E0789]: cannot call associated function on trait without specifying the corresponding `impl` type
   --> $DIR/associated-types-unconstrained.rs:14:20
    |
+LL |     fn bar() -> isize;
+   |     ------------------ `Foo::bar` defined here
+...
 LL |     let x: isize = Foo::bar();
-   |                    ^^^^^^^^ cannot infer type
-   |
-   = note: cannot satisfy `_: Foo`
+   |                    ^^^^^^^^ cannot call associated function of trait
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0283`.
+For more information about this error, try `rustc --explain E0789`.

src/test/ui/error-codes/E0283.rs

@@ -27,7 +27,7 @@ impl Generator for AnotherImpl {
 }
 
 fn main() {
-    let cont: u32 = Generator::create(); //~ ERROR E0283
+    let cont: u32 = Generator::create(); //~ ERROR E0789
 }
 
 fn buzz() {

src/test/ui/error-codes/E0283.stderr

@@ -1,10 +1,16 @@
-error[E0283]: type annotations needed
+error[E0789]: cannot call associated function on trait without specifying the corresponding `impl` type
   --> $DIR/E0283.rs:30:21
    |
+LL |     fn create() -> u32;
+   |     ------------------- `Generator::create` defined here
+...
 LL |     let cont: u32 = Generator::create();
-   |                     ^^^^^^^^^^^^^^^^^ cannot infer type
+   |                     ^^^^^^^^^^^^^^^^^ cannot call associated function of trait
+   |
+help: use a fully-qualified path to a specific available implementation (2 found)
    |
-   = note: cannot satisfy `_: Generator`
+LL |     let cont: u32 = <::Impl as Generator>::create();
+   |                     ++++++++++          +
 
 error[E0283]: type annotations needed
   --> $DIR/E0283.rs:35:24
@@ -27,4 +33,5 @@ LL |     let bar = <Impl as Into<T>>::into(foo_impl) * 1u32;
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0283`.
+Some errors have detailed explanations: E0283, E0789.
+For more information about an error, try `rustc --explain E0283`.